The present invention relates to a transport mechanism for repeatedly conveying an original to be copied past an optical scanning device in an electrophotographic copying machine of the type in which a leading edge of the original is continually carried by a gripping unit during the entire cycling operation. It particularly relates to a transport mechanism of the above-described type in which distortion is minimized and lower intensities of illumination are required than have heretofore been found necessary.
When an original is copied several times, the intervals for returning the original to its initial position after scanning and for delivering the original to the optical scanning device should be as short as possible. Machines which meet these requirements are disclosed in German Offenlegungsschrift No. 2 026 063 and U.S. Pat. No. 3,689,143. However, such machines perform optical scanning over a curved surface. If originals of widely varying sizes are to be copied in these machines, only very narrow scanning slots can be utilized since a high degree of transport mechanism utilization is required and the amount of distortion must be kept low. The use of narrow scanning slots in turn requires the use of high capacity illuminating equipment and/or a highly lightsensitive photoconducting material which often results in overheating of the apparatus.
The sheet conveyor mechanism known from German Offenlegungschrift No. 2 026 063 comprises a document drum, equipped with means for engaging the leading edge of an original to be copied, which conveys the leading edge of the original through the illumination and projection area of the reproducing machine by means of a driving arrangement, and then stops the leading edge at a predetermined point. After stopping of the leading edge, the trailing edge of the original is led past the illumination and projection station by conveyor belts provided in a lower region behind the document drum.
When the document drum is at rest the elastic conveyor belts exert a high pressure upon the original, in order to obtain an adequate frictional contact for further transport. Since the frictional force between the surfaces of the conveyor belts and the face of the original is counteracted by the frictional force between the back of the original and the document drum, a jerky movement results in the transition phase which may lead to a so-called "blurred" or shortened reproduction in which straight lines at a certain point show a slight distortion or contraction.
In U.S. Pat. No. 3,689,143 a sheet feeder mechanism is described in which a document drum transports the leading edge of an original through an optical scanning area by means of gripper fingers which engage the leading edge of the original. Optical scanning is performed on the curved surface of the document drum, which necessarily therefore requires the use of a narrow scanning slot, so that the problem of distortion and the problem associated with the use of high illumination intensities in the optical scanning area are presented.
Accordingly, there exists a great need in the art for a transport mechanism of the above-described type which reduces distortion to a minimum, provides a high degree of mechanism utilization when an original is repeatedly copied, and which eliminates overheating of the apparatus.